1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to an apparatus for continuously supplying lithographic printing material to the plate cylinder of a planographic printing press or a plate-material imager.
2. Background
Traditional techniques of printing an image onto a recording medium, such as paper, include letterpress printing, gravure printing and offset lithography. All of these printing methods require the use of plate material. This plate material is usually loaded onto a rotating plate cylinder that is brought into pressurable contact with the recording/printing medium.
In letterpress printing, the image is represented on the plate material as raised surfaces that accept ink and transfer it onto the medium. Gravure plates, in contrast, define a series of wells or indentations that accept ink for deposit onto the recording medium. Excess ink is removed from the plate material using a doctor blade or another similar device prior to contact between the plate and the recording medium.
In offset lithography, an image is defined on a printing plate defined by ink-accepting (oleophilic) areas surrounded by ink-repellent (oleophobic) surfaces. Two different lithographic systems are generally employed in offset lithography. In a dry printing system, the plate material is simply inked, and the image is transferred onto a recording/printing medium. First, the plate material makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other medium. The paper is typically pinned to an impression cylinder in rolling contact with the blanket cylinder, which applies ink to the paper in accordance with the image.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or “fountain”) solution to the plate material prior to inking. The fountain solution prevents ink from adhering to the non-image areas but does not affect the oleophilic character of the image areas.
In any lithographic system, accurate image transfer requires that the plate material remain relatively stationary on the cylinder surface during printing. Accordingly, different techniques have been developed for affixing plate material to underlying plate cylinders. Basic offset printing systems involve stationary clamping of a flexible length of plate material to the plate cylinder, while more advanced systems such as those described in U.S. Pat. Nos. 5,355,795 and 5,727,749 (both co-owned with the present application, and expressly incorporated herein by reference) use a relatively long length of plate material or web material stored in the form of rolls within a well or cavity in the plate cylinder. In these systems, a new segment of the plate or web material is advanced around the plate cylinder following completion of a print job. The new segment is imaged by an electronically controlled print head, which applies a print pattern to the surface. In these systems, it is critical that sufficient tension be maintained in the web material wrapped around the cylinder surface.